the core. If the core is to be removed through to the
impossible to remove paint without sanding into some
opposite skin of a nonuniform thickness sandwich,
of the load carrying ibers; however, the amount of
the core near the opposite skin should be cut by
iber damage should be kept to an absolute minimum.
hand with a core knife. The skin-to-core bond can be
Chemical paint strippers are unsatisfactory for paint
easily broken with peeling action. Cutters 1/4 inch in
removal as they attack the matrix material in composite
diameter may be preferred to 3/8 inch cutters because
laminates causing degradation.
of easier tool handling. Tungsten-carbide tools can
be used for most applications.
Hot bonders or other heat curing equipment
b. Drill Motor. Drill motors come in many vari-
must meet Class I Div II per 29 CFR 1926.449
eties. A 3500 rpm drill motor has been found to per-
for operation within 5 ft of aircraft fuel tanks
form best when drilling advanced composites. When
drilling through composite structures containing alu-
minum skins, a 2000 rpm drill motor works best. Both
errors. Exercise care using radios when
straight and 90 degree drill motors are required for
access to different areas.
g. Hot Bonders. A hot bonder is a programmable
heat and vacuum control unit, usually used in con-
c. Die Grinder. The die grinder is a high-speed
junction with a heat blanket to perform an elevated
motor with either a 90 degree or inline head. When
temperature vacuum bagged repair. Hot bonders con-
it is equipped with a silicon-carbide abrasive disc, it
trol curing temperatures according to a programmed
is useful for the removal of partial thickness damage
cure cycle by regulating the electrical power applied
to composite laminates and tapering (scaring) the
to the blanket at set times, temperatures, and rates.
perimeter of the damage removal areas. The die
The hot bonder receives temperature information from
grinder is also used for partial depth removal of
thermocouples (TC) that monitor repair area temper-
atures. Vacuum capability is provided through either
the use of a vacuum pump and regulator or a venturi
d. Kett Saw. The kett saw equipped with a dia-
system with manual adjustment of the vacuum level.
mond cutter blade is the quickest way to cut damaged
Temperature and vacuum alarms are used to indicate
laminates. The tool is easy to control, produces uni-
that an out-of-speciication condition exists during the
form cuts, and has a rapid material removal rate.
cure. The user sets the temperature and vacuum
The main disadvantage of the kett saw is that its
alarm limits when programming the hot bonder. Also,
use is limited to straight cuts or cuts with very slight
a printer that records the temperatures, vacuums, and
curves. Diamond coated tools are needed for boron
any out-of-speciication condition that occurs through-
and some carbon conigurations.
out the cure is useful for quality record purposes.
Aircraft hot bonders typically come with one or two
e. Reciprocating Saws. Saws that reciprocate
heating zones. Each zone has the capability of con-
the blade, commonly known as saber saws or Sawza-
trolling the temperature and vacuum separately from
llsŪ, should be avoided because the constant changing
one another, such that two hot bonded repairs may
blade direction imposes forces on the surface plies
be conducted simultaneously. The use of a dedicated
that tend to delaminate composites. These saws can
circuit is recommended for each hot bonder zone to
be either air or electric powered. Feed rates should
ensure adequate power exists to operate the heat
be slow to prevent heat buildup and the tool must be
held irmly to prevent loss of contact between the foot
and the material being cut. These saws are suitable
h. Heat Blankets. A heat blanket is a portable
for cutting mishap damaged parts into manageable
heat source normally used with vacuum bagged
sizes for containment and removal from the mishap
repairs. Figure 3-2 illustrates several types of heat
blankets. Heat blankets are typically regulated by a
heat control unit. This may be done automatically with
f. Sander. Sanding can be performed either by
a hot bonder. Folding heat blankets around sharp
hand or with power tools such as a pneumatic right
corners during use and/or storage can stretch or break
angle die grinder, Dual Action (DA) sander or jitterbug
the wire heating elements within the blanket. To min-
type sanders. Only currently approved paint removal
imize damage, store blankets lat or loosely rolled.
(and primer removal) method for composite ield repair
Heat is transferred from the blanket via conduction.
is sanding. Perform this method either by hand or by
Consequently, the heat blanket must conform to and
using a dual action/orbital sander. Use of other types
be in 100 percent contact with the repair area. This
of powered sanders is prohibited since there is a high
is usually accomplished using vacuum bag pressure.
potential for laminate damage. Use care during paint
Lack of contact between the heat blanket and the
removal to prevent sanding into the laminate. It is
area to be cured will result in inadequate heating.